1. Field of the Invention
The present invention lies in the field of the generation of extreme ultraviolet (EUV) radiation. It refers to an extreme ultraviolet light source according to the preamble of claim 1.
2. Discussion of Related Art
The next generation of semiconductor devices will be manufactured using extreme ultraviolet (EUV) lithography. EUV light is electromagnetic radiation with wavelengths between 120 nm and 10 nm. In EUV sources, a EUV-emitting plasma is produced by irradiating a target material, e.g., tin (Sn). The radiation exciting the target material can be a laser beam, thus producing a laser produced plasma (LPP). The EUV radiation is collected by collector optics, e.g., a collimating mirror, and directed to an intermediate region for utilization outside of the EUV light source.
The debris produced by the EUV emitting laser plasma limits the lifetime of the collector optics and should be mitigated in order to assure high volume manufacturing. Debris particles are dangerous, because they have a high kinetic energy and thereby cause erosion of the collector optics. Two kinds of debris can be identified: ions and neutrals. The former are the most dangerous for the collector, because they are energetic (1 to 10 keV) and charged, promoting chemical reaction when hitting surfaces. However, the ions can be deflected through an electric field (F. Alfieri, Optimization study and design of a laser plasma debris-free EUV collector module, Master Thesis, ETH, 2008). On the other hand, neutrals have a lower kinetic energy (<1 eV) and are less reactive, but are more difficult to mitigate. Another important challenge that regards the optics in the EUV source facilities is the thermal management of the collector, since the shape of the collector, hence the light focalization, is influenced by the thermal expansion.
WO-2008/105989 A2 discloses an LPP EUV light source. The light source comprises a chamber into which droplets of a material that is capable of radiating extreme ultraviolet light when excited into a higher energy state are delivered. The droplets are irradiated by light pulses generated by a laser, thus producing a plasma and generating EUV emission. The light source further comprises a collector optics for collimating the EUV light in an intermediate region for further use. WO-2008/105989 A2 suggests disposing a flowing gas between the plasma and the optics. The gas should establish a gas pressure sufficient to operate over the distance between the plasma and the optics to reduce ion energy to a target maximum energy level before the ions reach the optics. The gas is delivered into the chamber by a regulated gas source, flows through the chamber and is removed from the chamber by an adjustable pump.
WO-03/026363 A1 teaches to prevent debris accumulation on the optics by means of a gas curtain. A gas curtain device projects a stream of gas over the path of the
EUV light to deflect debris particles into a direction that is different from that of the path of the light.
WO-2009/025557 discloses a module for producing extreme ultraviolet radiation including a supply configured to supply droplets of an ignition material to a predetermined target ignition position and a laser arranged to be focused on the predetermined target ignition position and to produce a plasma by hitting such a droplet which is located at the predetermined target ignition position in order to change the droplet into an extreme ultraviolet producing plasma. Also, the module includes a chamber including a collector mirror that includes a mirror surface constructed and arranged to reflect the radiation in order to focus the radiation on a focal point FP and a fluid supply constructed to form a gas flow flowing away from the mirror surface in a direction transverse with respect to the mirror surface in order to mitigate particle debris produced by the plasma. The particle debris mitigation preferably occurs using the Peclet effect. The so-called Peclet number describes the rate of advection of a flow to its rate of diffusion, often thermal diffusion. It is equivalent to the product of the Reynolds number and the Prandtl number in the case of thermal diffusion, and the product of the Reynolds number and the Schmidt number in the case of mass dispersion. By creating a flow such that its advection is sufficiently high, the Peclet number will become so high, that the particle debris reaching the collector mirror will be sufficiently low. However, a disadvantage of this solution is the massive gas flow into the interior of the chamber, where the collected EUV radiation passes.